ORGANIC EVOLUTION 



181 



and in consequence the thick 

 dorsal wall of the body stands 

 in marked contrast with the 

 thin and soft ventral wall. 

 This axis is extended posterior- 

 ly into the tail, and expanded 

 anteriorly to form part of the 

 skull, which is the skeleton of 

 the head, and which may 

 readily be felt with the fingers 

 through the soft skin. Two 

 pairs of appendages are quite 

 characteristic of vertebrates. 

 The close correspondence be- 

 tween fore and hind limb will 

 be obvious even in the living 

 specimen. Both have a sup- 

 porting girdle of bone embed- 

 ded in the side walls of the 

 body and more or less firmly 

 attached to the axial skeleton. 

 Upper arm, fore arm, wrist and hand, in the fore limb, 

 correspond to thigh, shank, ankle and foot, respectively, of 

 the hind limb. The divisions between these joints may 

 readily be determined by flexing them between the fingers. 

 Were not this internal jointed skeleton, with its numerous 

 bones united by strong ligaments and moved upon each 

 other by the over lying muscles so familiar to us, its 

 mechanical fitness would be most impressive. 



Another small part of the skeleton, located in the ventral 

 wall in the region of the throat, is the hyoid apparatus (fig. 

 112). This is mainly cartilaginous, only the part that is 

 stippled in the figure being bony. The anterior fork sup- 

 ports the base of the tongue ; the postero-lateral arms curve 



FIG. 111. Diagram of a vertebrate 

 skeleton, x, skull; y, fore limb; 

 2, hind limb; /, 2, 3, in front, 

 clavicle, coracoid, and scap- 

 ula, composing the shoulder 

 girdle; /, 2, j, behind, ilium, is- 

 chium and pubis, composing the 

 hip girdle. 



